Concerns over the release of various organic compounds into the atmosphere as components of waste gas has led to more and more stringent demands on industry to reduce emissions. The Environmental Protection Agency, for example, has recently enacted provisions concerning the amount of alcohol(s) which may be released into the atmosphere, such as ethanol and other alcohols released as components of the waste gases produced and/or released during fermenting processes.
Fermentation involves the metabolism of organic compounds by microorganisms. Compounds that are produced through fermentation include carboxylic acids such as lactic acid and alcohols such as ethanol. Molds, yeasts, bacteria and streptomycetes are all employed in fermentation processes.
Fermentation plays a role in the preparation and/or processing of many foodstuffs, including alcoholic beverages, such as beer and wine, vinegar, bakery products and the like. As a result, waste gases produced in such preparation and/or processing may include significant levels of oxygen-containing VOCs, such as alcohols, ethers, esters and carboxylic acids.
The brewing of beer, for example, includes the step of fermentation of a farinaceous extract obtained from a starchy raw material, barley, in the form of a malt. Other starchy raw materials that may be employed as the starting material are corn, rice, wheat, oats, potatoes and mixtures thereof.
The process for brewing beer involves mashing the malted barley with water, filtering off the solids and boiling the resultant filtrate (called a wort) with hops. The wort is then cooled and yeasts added.
During malting, enzymes, such as amylases, are released from the barley that transform the starchy raw material into fermentable sugars. These fermentable sugars are subsequently converted to ethanol and carbon dioxide by yeasts added to the wort. The beer is then processed and matured before marketing.
Bakery products include perishable foods, such as breads and cakes, and dry baked goods, such as cookies, crackers, pretzels, ice cream cones and the like. The total value of the entire baking industry in the United States exceeds $11,000,000,000.
Many bakery products tend to be highly perishable foods. Much of the production of these goods is therefore done in small plants distributed throughout the United States. From the plants, distribution of perishable bakery products is made within a relatively limited area, unless the product is first frozen to preserve its freshness for a longer distribution period. According to the 1977 Census of Manufactures, these small plants account for almost 75% of the total baking facilities in the United States.
Dry baked products, such as cookies, crackers and pretzels, have a relatively longer shelf life and can therefore be manufactured at a large central location and distributed over a wide area. According to the 1977 Census of Manufactures, there were 24 of these large facilities preparing dry baked products, with a combined output of $2,800,000,000.
Ingredients in bakery products, both perishable products and dry baked products, generally include the following: flour; leavening agents; sugar, such as corn syrups, sucrose and dextrose; fats and oils, such as animal fats and vegetable oils; milk; eggs; salt; and flavoring and enriching ingredients. Leavening agents may be loosely classified into two general categories: yeast, such as fresh compressed yeast or active dry yeast, and chemical leavening agents, such as baking powders. Yeast-raised products make up nearly two-thirds of the total dollar volume of the baking industry.
Most breads and other yeast-raised products are prepared by a sponge-dough process which has four basic steps. First, a sponge of flour, water, yeast and sugar is mixed and allowed to ferment. The sponge is then mixed with any additional ingredients, such as fats or milk, to develop the gluten in the dough. Alternatively, liquid sponges, which contain less flour, may be employed.
After a rest period, the dough is cut and shaped and allowed to rise. The leavening action of yeast is based upon its ability to break down the fermentable sugars in the dough, forming carbon dioxide and ethanol.
Once the dough has been allowed to rise, it is baked. During baking, the ethanol in the dough produced by the leavening action of the yeast is released to the atmosphere.
Malt vinegar is produced by surface or submerged-culture oxidation of malted barley, optionally mixed with one or more other grains. Amylases released from the malted barley convert the starches to fermentable sugars, such as glucose and maltose. These sugars are readily fermented by Saccharomyces yeast.
In addition to the above, there are a great number of other common processes that also involve release of VOCs, in particular, oxygen-containing VOCs, into the atmosphere. For example, in flexographic printing processes emissions arise from organic solvents that may be present in the ink and from solvent that may be used for press cleaning. Emissions are limited in all cases to VOCs.
Solvents commonly used in flexographic printing include ethanol, isopropanol, N-propanol, hexane, toulene, isopropyl acetate, N-propyl acetate, glycols, glycol ethers. In publication flexo, amines and glycols are typical VOC compounds used. Amines and glycols are utilized in quantities that never exceed 5% of the ink formulation (prior to dilution with water). In packaging flexo, VOCs content may be higher than 5%, particularly if alcohols are used as cosolvents. In use at the press, the allowable VOCs in the diluted ink may be no more than 25% of the solvent portion.
Emission control strategies for flexographic printing processes typically include solvent recovery and catalysts or thermal oxidation incineration. Thermal oxidation relies on high temperature, e.g., 1400.degree.-1500.degree. F., turbulence and adequate retention time to facilitate the combustion of VOCs into carbon dioxide and water. Catalytic incineration, on the other hand, is a low temperature, i.e., 500.degree.-750.degree. F. process which relies on a catalyst to convert the VOCs.
In catalytic oxidation procedures, a major concern is the presence of potential poisons in a waste gas stream containing the VOCs to be treated. The nature of the poisons may vary from process to process. Poisons bind chemically to the active sites of a catalyst and destroys its catalytic activity. Only chemical or heat treatment can sometimes reverse the effect of poisons.
Sulfur is among the most ubiquitous potential poisons. Sulfur containing compounds originate from several sources, including sulfur containing organic compound present in fermentation processes or some printing inks. The most common source of sulfur comes however from combustion of natural gas or fuel oils. The trace amounts of odorous compounds containing sulfur present in the fuel result in the presence of sulfur oxides (SO.sub.2 and SO.sub.3) in the combustion products. Fuel fired baking ovens, natural gas fired dryers and direct fired furnaces present in most bakery and printing processes result in varying amounts of sulfur oxides in the waste gas stream. A fuel fired burner is most commonly used in catalytic oxidizers to heat-up the process gases before they reach the catalyst, which ensures that sulfur oxides will be present in the gas stream.
Although the amounts of sulfur in a waste gas stream is usually in the ppb range, its cumulative effect over time can be significant and can reduce the lifetime of a catalyst.
Thus, there exists a need in various industries for catalysts and methods which can efficiently reduce the level of VOCs, in particular oxygen containing VOCs, in waste gases at low temperature. There is also a need for oxidation catalysts useful for the destruction of VOCs which offer increased resistance to poisoning in the presence of sulfur.